1. Field of the Invention
The present invention relates to a method for determining an inversion time value of tissue using magnetic resonance technology, as well as to a magnetic resonance apparatus for implementing such a method.
2. Description of the Prior Art
In recent years, contrast agent-supported tissue characterization by means of magnetic resonance technology has found increasing distribution in clinical practice. Conclusions about the extent and degree of damage in undernourished tissue or, respectively, after infarcts can be obtained with the aid of contrast agent-supported magnetic resonance examinations.
The basic principle of contrast agent-supported magnetic resonance imaging for tissue characterization utilizes the kinetics of a contrast agent based on gadolinium in tissue. In infracted, necrotic or scarred tissue, the contrast agent enrichment takes significantly longer than in healthy tissue due to the poor perfusion. A hyper-intense depiction of poorly perfused tissue thereby ensues later and is maintained for up to 90 minutes after the contrast agent administration. The region of the diseased tissue can thus be very precisely delimited. A demarcation between dysfunctional but still vital tissue and infarcted tissue is also possible.
To control the contrast between healthy and diseased tissue, the inversion recovery (IR) magnetization method is used in the magnetic resonance sequences for imaging. In the inversion recovery sequence, the contrast between healthy and damaged or, respectively, undernourished tissue predominantly depends on the T1 relaxation time. In an inversion module activated before the actual imaging sequence, a preparation pulse or 180° radio-frequency pulse inverts the longitudinal magnetization. The transversal magnetization thereby remains equal to zero. During the subsequent repetition, the negative longitudinal magnetization decays to zero and then rises again. Since no transversal magnetization can be created, no signal is measured either. In order to be able to generate a magnetic resonance signal, the longitudinal magnetization must be converted into a transversal magnetization via a subsequent excitation pulse, for example a 90° excitation pulse.
One measurement parameter in this sequence is the point in time between the inversion pulse and the excitation pulse; it is called the inversion time TI. The contrast between different tissue types is set with the inversion time TI. However, there is no standard value for the inversion time TI that always allows an optimal contrast distribution of the different tissue types. Normally, the contrast is controlled so that normal and healthy tissue is suppressed in the image presentation, thus is shown dark.
An important application field of the inversion recovery sequence is the vitality examination of heart tissue and the characterization of heart tissue, thus of the heart muscle or myocardium.
In magnitude images, as was already mentioned above the contrast very strongly depends on the correct setting of the inversion time TI. Here the optimal setting has until now been determined with the aid of a TI scout measurement method. The TI scout measurement uses a CINÉ sequence in which an inversion pulse is radiated at a trigger point in time. The different acquired heart phases have different time intervals relative to this pulse. The time interval of the individual images from the trigger point in time is thereby identical to the inversion time TI. After the end of the acquisition, the user then visually assesses in which image the healthy myocardium is shown darkest. The time interval between the trigger point in time and the measured heart phase that belongs to this image corresponds to the optimal TI value for the magnitude images.
The visual evaluation of the CINÉ image sequence for the selection of the optimal TI value as well as a subsequent manual translation of this value into the actual measurement sequence can contain errors. An incorrect or suboptimal TI value may be selected, and the translation of this value into the measurement sequence may be incorrect.
In the late enhancement technique that is implemented approximately 10 to 30 min after a contrast agent administration, the image acquisition in principle always ensues in the same heart phase. The inversion module is then activated variably before the image acquisition, corresponding to the variation of the inversion times.
The individual determination of the optimal TI value is superfluous if TI-independent pulse sequences are used, for example the PSIR (Phase Sensitive Inversion Recover) sequence. In addition to the image data, an additional echo is read out with a small flip angle. This echo is measured in the following heart beat. In a wider scope, the image contrast is therefore independent of the precise selection of the inversion time value. However, in spite of this many users want to use the optimal inversion time in the magnetic resonance image acquisition in order to have the generally accepted magnitude image present in addition to the finding.
A method for segmentation of the myocardium in real-time magnetic resonance image series is described in United States Patent Application Publication No. 2007/0116339 A1. A Hough transformation for approximation of limits of a structure shown in the image is thereby used.
United States Patent Application Publication No. 2007/0092131 A1 discloses a method with which the center point of a point-symmetrical structure is determined. A threshold selection method based on a greyscale histogram is thereby applied.